Spread spectrum communications are commonly used in cellular networks that provide wireless service to mobile stations, such as wireless telephones. Such cellular networks typically operate in accordance with IS-95 CDMA or cdma2000 standards.
The signals transmitted by base stations in a spread spectrum communication system are spread by a pseudonoise (PN) sequence. For example, in accordance with IS-95 CDMA standards, the pilot signal transmitted by a base station is spread by a “short” PN sequence at a chipping rate of 1.2288 MHz, and the “short” PN sequence repeats itself every 26.67 milliseconds. With this chipping rate, one “chip” is approximately 0.8138 microseconds.
Each pilot signal transmitted by a spread spectrum communication system may be spread by the same short PN sequence but with a different phase or “PN offset.” In particular, the beginning of a PN sequence used to spread a pilot signal may occur at a particular time offset relative to a reference time, according to the time used by the spread spectrum communication system. The “PN offset” of the signal may then correspond to this particular time offset.
In accordance with IS-95 CDMA standards, each PN offset is defined by an index that is an integer in the range of 0 through 511. The actual PN offset is then found by multiplying its index by 64 chips. Thus, a PN offset of “0” means that the PN sequence begins at the reference time. A PN offset of “1” means that the PN sequence begins 64 chips after the reference time. A PN offset of “2” means that the PN sequence begins 128 chips after the reference time, etc. In this way, different pilot signals may be distinguished by their PN offsets.
As noted above, the PN offset of a pilot signal is defined with respect to a reference time according to the system's time. However, when the pilot signal is received by a mobile station, the mobile station may measure a different PN offset. The difference between the PN offset transmitted by a base station (the nominal PN offset) and the PN offset measured by the mobile station (the measured PN offset) can arise due to a transmission delay associated with the pilot signal.
The transmission delay can be a function of the distance between the base station's antenna and the mobile station. For example, a pilot signal will travel approximately 244 meters in one chip (assuming that one chip is 0.8138 microseconds). Thus, it takes about 6.6 chips for a pilot signal to travel one mile. This means that if the mobile station is trying to detect a pilot signal from a target base station that is one mile away, the mobile station will measure a PN offset that is 6.6 chips greater than the nominal PN offset of the target base station's pilot signal (assuming that the mobile station's time is perfectly synchronized with the system's time).
Once a mobile station has found a target pilot signal and has measured its PN offset, the mobile station may continue to monitor the target pilot signal. While monitoring the target pilot signal, the mobile station may subsequently measure a PN offset for the target pilot signal that is either greater than before (e.g., if the mobile station is moving away from the target base station) or less than before (e.g., if the mobile station is moving toward the base station). Because of the possibility of subsequently measuring an increased or decreased PN offset for the target pilot signal, the mobile station typically monitors the target pilot signal using a search window that is centered on a previously-measured PN offset. The width (in chips) of the search window is typically specified by a width parameter that the mobile station receives from the spread spectrum communication system. In this way, the mobile station may continue to track the target pilot signal when the mobile station changes its distance from the target base station, so long as the target pilot signal's PN offset at the mobile station remains within the range of PN offsets specified by the search window.